I've got a 9 speed back, 3 speed front. (Shimano Mega 9)

When in front middle and back bottom (well bottom 2 cogs on the back
really) the chain catches on the front derailleur, I'm not really sure
what I'm doing with regards to sorting it out...

I had some really good bike maintenance websites but I lost my bookmarks
a while back, can anyone recommend some?